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<head>
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <ul>
        <li>item 1</li>
        <li>item 2</li>
        <li>item 3</li>
        <li>item 4</li>
        <li>item 5</li>
        <li>item 6</li>
    </ul>

    <script> 
        // jquery 扩展选择器 :eq(index) index: 下标从0开始 选中第三个li标签

        $("li:eq(2)").css("border","2px solid red")
        
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